Vegetation trimming devices are well known and are commonly used to maintain one's lawn and garden. There are a number of different classifications of rotary trimmer heads, defined generally by the manner in which a monofilament trimmer line is fed, spooled or replaced in the trimmer head and/or how the trimmer line is discharged, indexed or lengthened during use of the trimmer head. Such classifications include (a) “self-indexing” trimmer heads, wherein trimmer line is automatically released from a spool inside the trimmer head upon a determination of increased rotational speed of the trimmer head, (b) “bump-fed” or “bump-activated” trimmer heads, wherein trimmer line is mechanically released from a spool inside the trimmer head by the user “bumping” the bump knob or similar member on the ground so as to permit a length of line to be released, and (c) “fixed-line” trimmer heads, wherein a length of line is introduced and loaded or otherwise attached manually through spaced openings usually, but not always, at the periphery of the trimmer head into a line-holding mechanism that holds the trimmer line to the trimmer head during use, eliminating the need for a spool. The present invention is directed to a special type of these “fixed-line” trimmer heads.
The majority of aftermarket trimmer heads sold are (c) “fixed-line” trimmer heads because they are easily designed to be universally adaptable to essentially any brand of rotary trimmer regardless of the operation of the trimmer head or the direction of rotation. Replacement strips of trimmer line used for this type of trimmer head are typically about 8 to 15 inch in length and can be easily added to the head through either openings at the periphery or within the line-holding mechanisms employed. Generally, fixed-line trimmer heads use a base portion having a series of radially spaced line holders, each capable of holding one strip of trimmer line. After the line strip wears, the consumer manually removes the used strip. Once the old worn length of line is removed, a new length of trimmer line can be loaded into the line holding mechanism. There is no indexing or lengthening of the line during use. The ease-of-use has been a reason for the market acceptance of this type of trimmer head. However, the need for improvement still exists for this type of trimmer head.
There are a variety of methods and mechanisms developed and discussed in the prior art for holding a fixed length of trimmer line to a trimmer head. Among the more popular methods for holding strips of line include (i) means for holding one end of a single strip of line such that one end is held by and within the trimmer head, while the other end impacts and cuts vegetation, (ii) means disposed within the trimmer head housing for holding the strip of line in the middle such that both distal ends extend from the trimmer head for impacting and cutting vegetation, and (iii) means for holding a folded strip of line outside of, and typically above, the trimmer head such that both ends of the strip of line impacts or cuts vegetation. Since the present invention is focused on improving fixed-line trimmer heads that employ line-holding mechanisms similar to those used to hold folded strips of line (i.e., method (iii) above), but with certain modifications, the prior art and problems associated with these line-holding mechanisms for folded strips of line are further discussed.
U.S. Pat. Nos. 7,603,782 and 7,743,511, the disclosures of which are incorporated herein by reference, disclose trimmer heads with various line holding mechanisms for holding folded strips of line. Unlike the other types of “fixed line” trimmer heads, the strips of line are attached to the line-holding mechanism outside of the trimmer head housing. That is, the line is attached to the line-holding mechanism above or below the trimmer head housing. Furthermore, the line-holding mechanism is capable of pivoting on a vertical axis for the purpose of reducing stress on the line, not for any purpose related to the clamping, or otherwise holding, of the line.
While there are several mechanisms disclosed for holding folded strips of line, the most commercialized of these line holding mechanisms is disclosed in the '511 patent, wherein the line-holding mechanisms have the basic shape of a round post with a flange at the lower end. The top end is beveled (slanted) on two opposite sides with two straight-through passageways for insertion of the trimmer line as best seen in FIG. 19 of the '511 patent. The flange is sandwiched between a lower cover and the upper portion of the head and is held such that it can rotate about a vertical axis but cannot be move vertically. The pivot posts on commercial trimmer heads are injection molded as a single piece of plastic and have two straight, parallel through holes or line channels with a portion of plastic separating the two through holes or line channels.
To load the trimmer line into the trimmer head, two opposed ends of a folded strip of line are threaded through the two straight parallel holes till the central folded portion of line is pulled tightly against the portion of plastic separating the two holes. Typically, this is done by loading one end of the folded strip of line through an entrance to one of the parallel, straight through line channels and simultaneously or in series, loading the other end of the folded strip of line through an entrance to the other of the parallel, straight through line channels, which entrance is adjacent the first line channel. The folded strip is then pulled tight until the folded central portion of the strip of line disposed against the portion of plastic separating the two line channels. Commercial trimmer heads utilizing this type pivoting line holding mechanism can have two, three or four pivoting posts and are commercially available from various commercial outlets and manufacturers under the tradenames PivoTrim® X2, PivoTrim®, the PivoTrim® Classic and PivoTrim® Pro, wherein PivoTrim is a registered trademark of Better Heads, LLC of Pearland, Tex., and Ugly Head®, wherein Ugly Head is a registered trademark of Shakespeare Company, LLC, of Columbia, S.C., respectively.
During use, the trimmer head is rotated by a trimming device such as a weed trimmer machine or brush cutter. The ends of each folded piece of trimmer line will extend out radially beyond the periphery of the trimmer head due to the centrifugal force created by the rotation of the trimmer head. The tips of the trimmer line are able to cut vegetation due to their velocity, due to rotation of the trimmer head. However, if the trimmer line impacts an immovable object or very dense vegetation, the line and post can pivot away from the point of impact, which reduces the stress on the trimmer line.
Unfortunately, line drag has been found to be problem with this type of “fixed line” trimmer head. Because each line in the prior art for this type of “fixed line” trimmer head is folded in half to form an extended “U”, each line is, in actuality, two lines cutting essentially the same vegetation at or near the same place relative to the trimmer head. If there four radially spaced line-holding mechanisms on a trimmer head, it will be appreciated that there are no fewer than eight extended lines, two for each line-holding mechanism, given that folded strips of line are required wherein the extended lengths from the fold to the end of the line are essentially equal in length. It has been found that, once one of the lines cuts the vegetation, the second line is not needed and only serves to slow the velocity of the tips of the lines due to wind drag. Further, it is also possible that the second line cuts vegetation that is already cut by the first line, again causing a reduction in the velocity of the entire line. Cutting vegetation that is already cut does not serve any identifiable purpose.
Thus, the need exists for a “fixed-line” trimmer head having a pivoting line-holding mechanism of the type similar to that used for folded strips of line, but which is capable of holding line that will lessen the drag upon the use of the line.
Because the present invention involves “fixed-line” trimmer heads for holding single strips of line in combination with pivoting mechanisms, the prior art with respect to this concept is also addressed. Specifically, both Iacona U.S. Pat. No. 7,257,898 and Aliss U.S. Pat. No. 8,307,558 teach methods for holding single strips of trimmer line in combination with pivoting posts. However, both of these mechanisms involve adding components, such as clamps and springs, to the internal portion of the pivoting mechanism. As a result, the housing of the mechanism must be enlarged to make room for the extra components. This makes the overall mechanism larger in size and adds weight. The increased weight from the larger post mechanisms and the added components is affected by the centrifugal force generated by the rotation velocity of the trimmer head. This increased outward centrifugal force increases the resistance to pivoting of the mechanism due to friction between the mechanism and the opening in the trimmer head. Thus, it is more difficult for the line-holding mechanism to pivot.
Likewise, the energy to pivot the line-holding mechanism is reduced due to the reduction from two legs to one leg of trimmer line extending from the trimmer head. The line, being operatively connected to the pivot mechanism, when impacted by an immovable object provides the energy to rotate the line-holding mechanism in one direction so that the line moves away from the object. The centrifugal force acting on the line then provides the energy to rotate the mechanism in the second direction. The centrifugal force makes the line pivot until it reaches an outward radial position. The amount of centrifugal force is dependent on the rotational speed and the mass of line. By reducing the number of end lengths extending from the mechanism from two per mechanism to one per mechanism, the centrifugal force is reduced in half. The energy associated with this reduced centrifugal force must overcome the increased frictional resistance associated with the heavier pivot post.
Thus, the need exists for a “fixed-line” trimmer head having a pivoting line-holding mechanism of the type similar to that used for folded strips of line, but which is capable of holding line that will lessen the drag upon the use of the line and which does not increase the weight of the line-holding mechanism.